Oh Chute

[Inyodog]

I working on my L2 rocket, I’m almost done…it’s looking good. The rocket is my own design, dual deploy, 4” dia, MagnaFrame, G10 fins, 78” long, and weighs 125oz, (without motor).
A 60” dia. std chute (Cd .8) brings it down around 20.4 ft/s. (per open rocket)
I’m trying to decide on what parachute to buy, and would like some input.
My options are:
60” sunward, octagon, (8 shroud lines) least expensive.
58” topflight, octagon, (8 shroud lines) good reputation… mid range cost.
60” Giant leap, round, (16 shroud lines) seems to be heavy duty… mid range cost.
58” LOC, round (10 shroud lines) most expensive.
My question is: since the descent rate should be about the same for all of them assuming the Cd is .8), does the shape and number of shroud line make a difference? Would the one with 16 lines tend to get more tangled and less reliable than the one with 8 lines? Would more shroud lines increase the Cd? Anything else I should consider?
By the way, For now I'm not considering the expensive hemi or elipical types now, I may look at those next.

 

[dhkaiser]

The Top Flight 58" has a CD of 1.34 according to the owner Gary.

"The CD Par9-Par24 1.09
Par-30-58" 1.34
Par70-120" 1.40"

 

[pythonrock]

They are all different designs with different CDs, you can't just go by the listed size.
Here is a great site for comparing descent rates of chutes from several manufacturers.
https://fruitychutes.com/help_for_parachutes/parachute-descent-rate-calculator.htm

 

[MikeyDSlagle]

They are all different designs with different CDs, you can't just go by the listed size.
Here is a great site for comparing descent rates of chutes from several manufacturers.
https://fruitychutes.com/help_for_parachutes/parachute-descent-rate-calculator.htm

Looks like you have to manually enter the Cd, but if you have that then,yes, that site is helpful. Every chute manufacturer should have the Cds listed on their site.

More shroud lines may increase the chance of tangling if for no other reason than being a PITA to fold correctly.
Of those you listed, I've only used Toplight so my knowledge is limited.

Topflight has lots of color options if that is important to you. And Gary will do custom work as well so if you need an oddball size..

 

[Inyodog]

I agree, different designs, hemispherical, elliptical, X form, flat sheet etc would have different Cd's. I'm mainly concerned with flat parasheets.
I looked at that calculator, it's very good. They show a Cd of .75. for a Top Flight par-58. I believe a of Cd.75 is more realistic for flat parasheet than the Cd of 1.34 as stated above. No matter what shape the parasheet is be it octagon or round, I don't see how the Cd can very that much.

 

[crossfire]

All I can tell you is our chutes were tested way back when we first opened TFR by a friend who worked for NASA. I have always trusted his tests.

Gary

 

[dhbarr]

Cd of a parasheet is reasonably affected by the number and length of shrouds. Does anyone have a publication showing the general relationship ( e.g. optimizing length & number vs. descending mass )?

 

[dhkaiser]

I agree, different designs, hemispherical, elliptical, X form, flat sheet etc would have different Cd's. I'm mainly concerned with flat parasheets.
I looked at that calculator, it's very good. They show a Cd of .75. for a Top Flight par-58. I believe a of Cd.75 is more realistic for flat parasheet than the Cd of 1.34 as stated above. No matter what shape the parasheet is be it octagon or round, I don't see how the Cd can very that much.

I flew a TFR 36" that was simmed to CD.8. Came down slower than predicted. Adjusted the CD to 1.34 and it gave a spot on descent rate.

 

[Coop]

In a flat sheet, yes, the number of lines will make a difference... it relies on the lines to assume the semblance of a cupped chute--the more lines, the closer approximation it can make. There's a limit, though--as they typically do not have a vent hole, so they're only going to catch so much air before it starts bypassing around the chute mouth. Also keep in mind the weight rating per line...

The length of the shroud lines will typically have effects on the chute's opening characteristics.

The flat sheets will often do the job for single-deploy, but for dual-deploy I'd want a real gored chute... with these, you typically get four layers of material at the line attachment points (more, if reinforced there), and thus, will be stronger. Other options besides the sphericals and ellipticals are the X- form types, the Skyangle cupped parabolics, the Rocketman dragster styles, as well as the toroids. These will be more expensive in a given size than a flat sheet, but they're also more efficient, so smaller chutes can be used. For example --in the weight range you're talking about, my last chute was 48" diameter, and recovered 7ish lbs at 15 FPS.

Later!

--Coop

 

[crossfire]

I can tell you for sure there has been of thousands of flat style chutes used for DD with no problems. With a rolled them like we use there are 4 layers of fabric for line attachment. I have seen many gored or other style chutes strip the lines. You don't need a high priced chute to get the job done right.

 

[markkoelsch]

The 58" Top Flight is a very solid choice. You could also look at the Top Flight Crossfire 48". Both are really nice chutes, and I speak as a friend of Gary's- if you need support you will get it from him. He is a top notch manufacturer and guy.

 

[Bat-mite]

I use a TFR in my Radial Flyer, which I got my L3 with, by the way. Descent weight is usually around 11 pounds, depending on motor. I'm landing on grass, and I have slightly upswept fins.

What is the descent weight of your rocket?

 

[Inyodog]

Looks like 8.5 lb

 

[Bat-mite]

Depending on your landing surface, sounds like the TFR 58" is plenty for you.

 

[davdue]

+1

I can tell you for sure there has been of thousands of flat style chutes used for DD with no problems. With a rolled them like we use there are 4 layers of fabric for line attachment. I have seen many gored or other style chutes strip the lines. You don't need a high priced chute to get the job done right.

I have used only Top Flight chutes on all of my cert flights. The only failure I have had was on my L3 attempt and it was too little BP. The airframe didn't separate at apogee and when the main deployed ballistic at 600 ft on a 25# rocket the shock cord loop ripped open. The 10' chute still opened enough to flip the rocket around so it landed fin can down. The fin can was saved and I will fly it again next year for my second attempt. The parachute got really tangled and I think one or two shroud lines ripped. Gary repaired everything for like $25. Not bad for a rocket coming in ballistic from 8000 ft. I have one of his cross fires in 60" and it opens fast and then comes down a little faster than the PAR's which is good for our windy days in Kansas.

The only chutes I own that are not Top Flight came with kits from Mad Cow or Public Missiles.

 

[crossfire]

Dave and Mark and all thanks for the kind words. Who do you think makes the MC chutes?

 

[Handeman]

The flat sheets will often do the job for single-deploy, but for dual-deploy I'd want a real gored chute... with these, you typically get four layers of material at the line attachment points (more, if reinforced there), and thus, will be stronger.
--Coop

I understand what you are saying and the multi folds of the hem will give you a stronger attachment point for the shroud line, but it probably isn't needed, even with the speed of deployment at DD main deploys. My brother was a licensed parachute rigger and told me that the heavy re-enforcements are not needed for our model rockets deploying at 100 ft/sec. He said that was way to slow and the weights were way too low to need the strength of man-rated attachments.

Of course I ignored his advice when I built my 9' chute for my L3, but if I build another, I'll follow that advice. Part of that is because I've got +50 DD flights using umbrellas skins as main chutes with about 1 1/2" of shroud lines zig zag to them and I've never had a shroud line tear loose. The only failed chute is one that was on a motor deploy rocket that deployed at motor burnout. The non-ripstop nylon of the umbrella shreaded, but none of the shroud lines let loose.

I understand why people want chutes that are sewn with lot of re-enforcement and look very strong, but for normal deployments, I don't think that is really needed. It is nice when the chute survives an anomalous deployment like at motor burnout or a failed apogee charge and ballistic deployment, but it might actually be better for the chute to fail and shread, acting as a streamer, then zippering the BT so bad because it holds together. That was certainly the case for my deployment at motor burnout. The chute shreaded and there was only a 2" x 3" triangle broke out of the phenaloc body tube.

Just some thoughts.... YMMV